The present invention relates in general to ink jet printers and more specifically to a low-cost printing system including a novel printhead and ink composition that provides photographic-quality resolution output.
Ink jet printers are popular and common in the computer field. These printers are described by W. J. Lloyd and H. T. Taub in xe2x80x9cInk Jet Devices,xe2x80x9d Chapter 13 of Output Hardcopy Devices (Ed. R. C. Durbeck and S. Sherr, San Diego: Academic Press, 1988) and U.S. Pat. Nos. 4,490,728 and 4,313,684. Ink jet printers produce high-quality print, are compact and portable, and print quickly and quietly because only ink strikes a print medium (such as paper).
An ink jet printer produces a printed image by printing a pattern of individual dots (or pixels) at specific defined locations of an array. These dot locations, which are conveniently visualized as being small dots in a rectilinear array, are defined by the pattern being printed. The printing operation, therefore, can be pictured as the filling of a pattern of dot locations with dots of ink.
Ink jet printers print dots by ejecting a small volume of ink onto the print medium. These small ink drops are positioned on the print medium by a moving carriage that supports a printhead cartridge containing ink-drop generators. The carriage traverses over the print medium surface and positions the printhead cartridge depending on the pattern being printed. An ink supply, such as an ink reservoir, supplies ink to the drop generators. The drop generators are controlled by a microprocessor or other controller and eject ink drops at appropriate times upon command by the microprocessor. The timing of ink drop ejections typically corresponds to the pixel pattern of the image being printed.
In general, the drop generators eject ink drops through a nozzle or an orifice by rapidly heating a small volume of ink located within a vaporization or firing chamber. The vaporization of the ink drops typically is accomplished using an electric heater, such as a small thin-film (or firing) resistor. Ejection of an ink drop is achieved by passing an electric current through a selected firing resistor to superheat a thin layer of ink located within a selected firing chamber. This superheating causes an explosive vaporization of the thin layer of ink and an ink drop ejection through an associated nozzle of the printhead.
The resolution of an ink jet printer is directly related to the size and number of ink drops printed on a print medium. For example, for a given area a small number of large ink drops produces a relatively low-resolution printed image while a large number of small ink drops generally produces a higher-resolution printed image. The quality and resolution of printed images that a printer is capable of producing are often compared to photographs, and xe2x80x9cphotographic-qualityxe2x80x9d resolution means that the resolution approaches that of a photograph.
There is a continually increasing demand for low-cost ink jet printers that are capable of producing xe2x80x9cphotographic-qualityxe2x80x9d images. Achieving this high resolution while keeping costs low requires a careful balance between the architecture of the printhead (such as the architecture of the firing chamber, the firing resistor and the firing frequency) and the composition of liquid ink. Typically, a change in the printhead architecture or in the ink composition to solve one problem may create other problems. Thus, in order to produce an inexpensive ink jet printer capable of photographic-quality resolution several factors in the printhead architecture and ink composition should be taken into account.
For example, in order to produce photographic-quality resolution, an ink jet printer typically will use a printhead architecture that increases the number of drops per area (the dot resolution). This increased dot resolution generally is accomplished by greatly decreasing the size of each drop. However, one disadvantage of decreasing the drop size is that the speed of the printer is decreased because the area of print media covered by a single ink drop is also decreased.
Although this reduction in printer speed may be mitigated by increasing the number of drop generators in a given area (drop generator density) on a printhead, this creates other problems. In particular, an increase in the drop generator density means that the drop size must be reduced and the frequency of drops ejected (the firing frequency) increased. However, this reduction of drop size and increase in firing frequency creates a drop instability whereby the velocity and the volume of each drop has the undesirable characteristic of varying or decreasing over time. A decrease of drop velocity over time (during a single firing burst) is known as decel. Decel and other changes in drop velocity are undesirable because the accuracy of the drop placement on the print media is adversely affected.
As such, producing a low-cost ink jet printer capable of generating high-resolution printed output usually requires a careful balance between several factors. A change in printhead architecture that solves one problem may create other problems. Therefore, there exists a need for a low-cost ink jet printing system that provides high-resolution (such as photographic quality) output with a suitable printhead architecture and ink composition.
To overcome the limitations in the prior art as described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention is embodied in a low-cost printing system including a novel printhead architecture and ink composition that provides photographic-quality resolution output. The printhead architecture and ink composition of the present invention enables the manufacture of an inexpensive printing system that provides high-speed, photographic-quality printing.
The printhead architecture of the present invention includes a high-density array of drop generators that eject ink drops having a low drop weight. The speed of the printing system is maintained by operating the printing system at a high operating frequency. Power and thermal issues arising from operating a large array of drop generators at a high operating frequency are alleviated by lowering the energy dissipated from each firing resistor and the energy required by each firing resistor to eject an ink drop. In particular, high-resistance firing resistors are used to provide minimum energy dissipation and these firing resistors are preferably thin-film resistors to ensure that a minimal amount of energy is needed to eject ink drops. The stability of the ink drop volume is maintained by including an overdamped structure within the ink drop generators that allow the ink chambers to fill up with ink slowly.
The ink composition of the present invention includes an ink additive that prevents decel from occurring during sustained high-frequency printing bursts. This ink composition includes an aqueous vehicle and a decel-alleviating component that is capable of undergoing a rapid thermal decomposition upon heating. This combination of printhead architecture features and ink composition provides high-speed, high-resolution printing within a low-cost printing system.
Other aspects and advantages of the present invention as well as a more complete understanding thereof will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. Moreover, it is intended that the scope of the invention be limited by the claims and not by the preceding summary or the following detailed description.